The dark color of menstruation is mainly due to low menstrual flow, combined with the dark brown or black color of the blood under the effect of the acidic environment of the vagina. There are many causes of low menstrual flow, including endocrine causes and uterine causes. Endocrine causes are big concepts, such as hypothalamic dysfunction, ovarian dysfunction, and thyroid dysfunction, which can cause endocrine disorders, which in turn cause low menstrual flow. Causes of uterus include congenital developmental factors such as small uterus. Acquired factors such as damage to the endometrium after abortion causing low menstrual flow, poor scar repair after cesarean section can also cause low menstrual flow, prolonged periods, low menstrual flow and dark color, it is recommended to go to the hospital for a comprehensive examination to see what the causes of low menstrual flow are.